1. Field of the Invention
The present invention relates to differentials, and, more particularly, to limited slip differentials.
2. Description of the Related Art
It is known that in vehicles with laterally displaced wheels, when such a vehicle makes a turn the outer wheel(s) must move further than the inner wheels and hence a direct coupling between the outer and inner wheels forcing the wheels to rotate in unison, necessarily induces some slippage between the wheels and the support surface during a turn. On the other hand, when more than one wheel receives a driving force, it is desirable that that force be transmitted as directly as possible to all of the driven wheels. One longstanding solution to this dilemma has been a conventional differential mechanism interconnecting a pair of driven wheels on a vehicle wherein each wheel is directly coupled by way of an axle shaft to a side gear within a differential housing. The side gears are in turn interconnected by a pair of beveled gears fixed by way of a cross pin to a differential case. The differential case receives the driving torque from the vehicle engine and during linear vehicle motion the differential case and axle shafts rotate in unison with no relative motion between the side gears or pinion gears. During a turning maneuver, one side gear associated with the outer wheel turns faster than the side gear coupled to the inner wheel while both wheels continue to receive driving force.
Such a conventional differential adequately solves the problem of wheel slippage on dry pavement or other comparatively hard surfaces; however, in slippery conditions such as snow or mud where one of the two driven wheels begins to slide or freely rotate, essentially no torque is transmitted to the other of the pair of driven wheels and the vehicle is stuck simply because one of its two driven wheels is allowed to slip.
One solution for preventing the problem of transmission of zero torque to one of the driven wheels is to provide a so-called "limited slip differential" which ensures that each of the wheels receives a certain ratio of the total torque applied to the wheels. An example of such a limited slip differential is disclosed in U.S. Pat. No. 4,612,825 (Engle), which is assigned to the assignee of the present invention and expressly incorporated herein by reference. In general, a pair of side gears are attached to respective cone clutches, which in turn engage clutch surfaces formed in the interior of the differential casing. An axially biasing device disposed between the side gears provides an axial preload on the side gears, which in turn effects a certain frictional resistance between the clutch surfaces of the cone clutch and the differential casing. The clutch surfaces of the cone clutch and casing are disposed at a 12.5.degree. angle relative to the longitudinal axis of the casing, and as a result provide a certain frictional resistance which must be overcome upon initial rotation of the differential to effect relative rotational movement between the cone clutches and casing. The axially biasing device disposed between the side gears includes a plurality of compression springs which apply a total preload force of between 1,000-1,500 pounds to the axial end faces of the side gears.
Although a step forward in the art, the limited slip differential disclosed by the '825 patent has a relatively low bias ratio of about 2.0. The bias ratio is defined as the ratio of the torque applied to the higher torque wheel divided by the torque applied to the lower torque wheel. A higher bias ratio means that the axles attached to the differential act as a locked axle, i.e., there is no relative rotation between the axles when a lower total torque value is applied to the wheels. This can be advantageous in certain applications, such as sports cars. For example, the driven axles will lock together at a lower total torque value and result in each wheel having equal torque applied thereto by the differential.
A problem with increasing the bias ratio of a differential is that the frictional resistance between the clutch surfaces is increased, thereby requiring a greater torque by the differential to overcome the frictional resistance. This means that a portion of the available torque is lost due to the increased frictional resistance.